The present invention relates to a method of treating and utilizing liquid flows at a chemical pulp mill, where wash filtrates are purified at an effluent plant of the chemical pulp mill comprising at least biological purification.
The size of chemical pulp mills has grown intensively during the last years, as today a pulp mill producing 1 million ton/a is of normal size and it does not seem that the growth of the size of pulp mills would be ceasing. At the same time that the size of the pulp mills is growing, the mills are being built in areas and surroundings with very strict environmental regulations. For example, the amount of water used by a mill is being continuously restricted. Because the size of the mill grows, minor decreases in the water amounts used by the mill per one ton of pulp do not absolutely decrease the amount of water used by the mill, but the amount is compensated back to the same level as the production size increases. This development is troublesome especially in countries where the amount of water available for the mill simply is not enough. In this kind of situation it is simply impossible to build a mill at a place where other demands of production are easily fulfilled, but due to water resources it is not possible to build a mill. Additionally, in many areas a cleaner environment is desired in such a way that the mills produce substances that are less detrimental to the environment. Therefore, it is essential to look for solutions for finding an increasingly closed process.
Chlorine-containing chemicals have been used throughout the production of chemical pulp in several different forms, of which elemental chlorine Cl2, chlorine dioxide ClO2 and hypochlorite NaOCl or CaOCl are the best known. Chlorine-containing chemicals have been used also e.g. in the form of hypochlorous acid in bleaching, but no permanent applications have remained in use. On the other hand, the chemical pulp industry has desired to tightly maintain a technique in which pulp is bleached with chlorine-containing chemicals so that chlorine dioxide is the main chemical of the bleaching process of the mill. Years-long pressure to reduce the amount of organic chlorine compounds in bleaching effluents has led to the point that first the use of chlorine and hypochlorite was abandoned and further the kappa number of the pulp after digestion was decreased from level 30 to level 10-15 for soft wood and from level 16-20 to level 10-13 for hard wood using an oxygen stage. In 1990s, the aim was to abandon the use of chlorine dioxide as well and many mills switched to the use of total chlorine free (TCF) bleaching technique, wherein the use of chlorine dioxide, too, was replaced by totally chlorine-free bleaching chemicals, such as ozone and peroxide. With this technique, all chlorine-containing chemicals could be avoided, but on the other hand, many paper producers were not satisfied with the properties of pulp produced without chlorine chemicals. Therefore, the marginal term for all solutions relating to the closing of the mill is that chlorine dioxide is still used as bleaching chemical.
Thus the dominating position of chlorine dioxide as bleaching chemical has even gained more power during the last years, and not even the latest researches or industrial experiences have managed to destabilize its position, but as a rule the whole pulp industry, with only a few exceptions, has approved the use of chlorine dioxide as the key chemical in bleaching. Thus, if a mill is to further decrease the amount of organic chlorine compounds, the aim of the mills will be, first and foremost, to eliminate them and to treat them inside the mill, rather than to decrease the use of chlorine dioxide.
Modern ECF-bleaching used for bleaching pulp, is typically formed of at least three bleaching stages and three washing apparatuses. In a special case there may be only two washing apparatuses, but such applications are rare. ECF-bleaching covers all such bleaching sequences, which have at least one chlorine dioxide stage and which do not use elemental chlorine in any bleaching stage. Because the use of hypochlorite is due to pulp quality reasons restricted to the production of only a few special pulps, such as dissolving pulps, also hypochlorite is not regarded to be used in the production of ECF-pulp, but it is not totally ruled out. Additionally, the bleaching sequence comprises one alkaline stage, wherein the additional chemicals used are today typically either oxygen, peroxide or both. Further, modern bleachings may use ozone, various types of acid stages and a chelate stage for removing heavy metals. In literature, the bleaching stages are described with letters:
O=oxygen delignification
D=chlorine dioxide stage
H=hypochlorite stage
C=chlorination stage
E=alkaline extraction stage
E0=alkaline extraction stage using oxygen as additional chemical
EP=alkaline extraction stage using peroxide as additional chemical
EOP(PO)=alkaline extraction stage using oxygen and peroxide as additional chemical
P=alkaline peroxide stage
A=acid hydrolysis stage, stage of removal of hexenuronic acids
a=pulp acidation stage
Z=ozone stage
PAA=peracetic acid stage, acid peroxide stage
In this patent application the chemical amounts and other amounts are given per one ton of air dry pulp (adt pulp, i.e. air dry metric ton of 90% dry chemical pulp).
When bleaching is called ECF-bleaching, the amount of chlorine dioxide used in the bleaching sequence is more than 5 kg act.Cl/adt pulp. If chlorine dioxide is used in one bleaching stage, most typically the doses are between 5-15 kg act. Cl/adt. The doses refer to active chlorine, whereby when converting to chlorine dioxide the dose has to be divided by a ratio of 2.63.
If the use of peroxide in bleaching is restricted to doses smaller than 6 kg and if chlorine dioxide is the main bleaching chemical, so then the chlorine dioxide dose in the bleaching increases from a level of 25 kg/adt depending on the bleaching properties of the pulp and on how much the kappa number of the pulp has been decreased before starting the bleaching using chlorine-containing chemicals. Thus, the bleaching technique may in view of the process be fairly freely adjusted to various levels of chlorine dioxide consumption so that the amount of chlorine-containing chemicals exiting the bleaching corresponds to the capacity of the chemical cycle to receive chlorides.
In connection with the present invention it is in view of practice most preferable to choose as reference sequence for hard wood a bleaching sequence ND-EOP-D-P effected with four bleaching stages and leave ozone out. The corresponding sequence for soft wood is D-EOP-D-P. Then the quality of the pulp can be regarded to correspond to the qualities required from ECF-pulp and the pulp yield remains reasonable. Then the chlorine dioxide doses for soft wood are typically between 25-35 kg/adt pulp and for hard wood 20-30 kg/adt. These values can be regarded as design values, and there is no need to invent any new specific techniques for bleaching. The theory of bleaching and various connection alternatives render a possibility for countless different bleaching sequences starting from the connection of two washing apparatuses up to six-stage bleaching sequences. At the same time, the number of chlorine dioxide stages may vary from one up to four and therebetween are alkaline stages as appropriate.
When the amount of active chlorine is calculated as described above in form of the chloride amount, it is noted that even with soft wood, for obtaining a good bleaching result, the bleaching line produces about 10 kg of chlorides per one ton of pulp and a hard wood bleaching line even less. If the plant is closed such that less and less of fresh water is led into bleaching, there may be a need to prepare for chlorine dioxide doses of even 50% greater, and on the other hand the amount of chlorides in bleaching effluents increases up to a level of approximately 15 kg, meaning that in practice the greatest doses of active chlorine are 60-70 kg/adt. Values higher than this cannot be considered economically reasonable, but the basic bleaching solution complies with these starting points. On the other hand, the aim of the present invention is to introduce an alternative, with which the closing of bleaching does not essentially increase chemical consumption.
One suggested technique for decreasing the environmental effects of chlorine-containing chemicals is the closing of the liquid cycles of bleaching plants, and modern bleaching plants have reached to a level of 10-15 m3 of effluent/adt pulp without a decrease in pulp quality. Nevertheless, even when decreasing the amount of bleaching effluent from a level of 15 m3/adt pulp to a level of 10 m3/adt an increase in chemical consumption is seen, which thus leads to an ever increasing amount of organic chlorine compounds from bleaching. Thus, a conclusion may be drawn that the closing of the water cycles of bleaching as such does not have a direct influence in the amount of organic chlorine compounds; but on the other hand a smaller amount and a greater concentration of effluents allow for easier and more economical purification thereof.
Chloride-containing chemicals are used in bleaching so that the total chloride dose into the bleaching plant is 5-10 kg of chlorides per one ton of chemical pulp. Because this amount has to be made to pass so that the amount of liquid to be evaporated in the process remains reasonable, the challenge is to find such a process arrangement, where a chloride-containing liquid replaces some other liquid used in a process at the mill. Thus there is no need for separate treatment stages, new non-productive sub-processes at the mill, but the treatment can be carried out by means of existing process stages.
In order to be able to optimize the treatment of a chloride-containing liquid and in practice the treatment of bleaching effluent, it is inevitable to first know the properties of the effluent. In the bleaching, chlorine-containing inorganic compounds and organic chlorine compounds from the reactions of chlorine dioxide or chlorine remain in the process. Bleaching separates from the fibers various compounds of lignin, which remain in the effluent in form of organic molecules. Additionally, sulfuric acid is used in bleaching for pH regulation and as main chemical in the hydrolysis of hexenuronic acids. Sodium hydroxide is also used for pH regulation and lignin extraction in alkaline stages. In addition to these, depending on the bleaching sequence, oxygen and peroxide are used in bleaching, which, however, are in elementary analysis such substances that their contribution in for example purification processes is not noticed. In some special cases, also hydrochloric acid may be used in pH regulation and sulfur dioxide or other reductants in elimination of chemical residuals from the bleaching, i.e. in elimination of unreacted bleaching chemicals.
Closing of the bleaching is based on recirculation of filtrates of washing apparatuses from later bleaching stages to preceding stages. The bleaching is planned only for circulating filtrates between bleaching stages and pulp from one stage to another to react with different bleaching chemicals. Thus, closing the whole bleaching is as an idea based on the fact that all substances separated in bleaching end up in filtrates. Optimizing the closing of bleaching is in a great part based on the way how reaction products of bleaching disturb the process of bleaching. Although in many various connections it has been stated that different degrees of closing are possible, practical experience has shown that such washing water arrangements of bleaching where the filtrates are connected so that the amount of waste water is less than 12-13 m3/adt increase the consumption of bleaching chemicals. Naturally, the quality of the pulp and the construction of the bleaching plant dictate the amount of additional chemicals used in the bleaching as the effluent amount of the plant decreases below the above presented level.
U.S. Pat. No. 5,470,480 presents a method with which e.g. an effluent flow of a chemical pulp mill is treated so that hydrogen peroxide is added thereto and the flow is exposed to ultraviolet radiation for forming of hydroxyl radicals from the peroxide. Hydroxyl radical oxidize organic impurities in the effluent flow in order to reach a desired purity level e.g. in form of chemical oxygen demand, COD, or expressed in form of color. The treated effluent flow can be recirculated back to the chemical pulp mill e.g. to pulp washing in bleaching. The color of the effluent is reduced in the method remarkably, as measured by Color Method EPA 111.2, to below 500, most preferably to below 20. According to an embodiment, unpurified acid effluent is added to the water thus purified, the mixture is treated in an oxidation pond, after which a portion of it can be circulated to bleaching.
EP863113 discloses a method, in which the alkaline filtrate of bleaching at a chemical pulp mill is treated (i.a. by means of ultrafiltration) so that an alkaline concentrate is formed that contains abundantly of organic compounds having a high molecular weight, and a flow from where the organic compounds have been removed. Said fractions can be used in brown stock washing. Said fractions can be used in brown stock washing. Recovery of these fractions allows decreasing the amount of AOX being removed from the bleaching plant to an effluent treating plant or to a surrounding water system. The acid effluent is treated in biological effluent treatment, in order to obtain the desired AOX, COD and color values for the effluent of the mill to the environment.
Publication Fontanier, V., et al. (“Simulation of Pulp Mill Wastewater Recycling after Tertiary Treatment”, Environ. Technology, 2005, Vol. 26, s. 1335-1344) has studied the circulation at a chemical pulp mill of effluent treated in a biological treatment plant and effluent that has not been treated. It was stated in the study that said effluent is to be further treated in a tertiary stage for preventing the increase in bleaching chemical consumption and the decrease in pulp brightness. The most efficient tertiary treatment was catalytic ozonization. Effluents were recirculated to E0-stage, to purification and pressing after D2-stage. The tertiarily treated effluent produced almost the same COD-contents as the use of clean water. The use of untreated effluent from the biological purification, plant, in its turn, led to high COD-amounts, which can cause excess consumption of bleaching, chemicals (e.g. NaOH in the E0-stage) and loss of pulp brightness. Untreated biological effluent has high COD- and color levels, whereas the corresponding values of effluent treated by catalytic ozonization are low. The use of untreated effluent is said to possibly cause damage at several points of the process due to the high COD-levels. That is, also according to this publication, liquids used in pulp treatment should be liquids with a purity level close to that of clean water.
The brown color of effluents is mainly of organic origin, especially from lignin decomposition products that are formed in different stages of pulp cooking and bleaching. Other substances that produce color are wood extractives as well and tannins and resins. Decolorization of effluents prior to leading them to a surrounding water system is considered important, because it is considered to have a detrimental influence on the living organisms and plants of the water system. According to said publications, the effluents are to be decolorized also before re-use in a pulping process for obtaining pulp of good quality.
WO-patent publication 2008152185 (FI-application 20080144) discloses a method with which purified effluent in an amount of at least 1 m3/adt pulp is introduced into dilution after a press or washing press, which effluent is passed from the dilution into the first process stage of bleaching. Preferably the effluent has been biologically purified for decreasing the lignin-content.
WO-patent publication 2008152186 (FI-application 20080298) describes a method with which more than one treatment line is arranged at the effluent purification plant for the mill effluents and effluents with different chemical compositions are purified in separate treatment lines so that the quality and amount of purified water from each treatment line is suitable for use in a stage or stages of the production process, whereto purified effluent is returned. With this method one or more filtrates of a bleaching sequence can be taken into a purification treatment and returned typically as washing or dilution water to bleaching and/or brown stock washing. The object of use of purified effluent is an object where this purified effluent is most suitable in view of its composition, such as chemical composition. Also in this method the effluent has been biologically purified for decreasing the lignin-content. The lignin-content of effluent decreased without dilution by at least 30%, preferably over 40%, most preferably over 60%.
The standard defined by the Technical Association of Pulp and Paper Industry Tappi, where e.g. the limit for color is very low, is still considered the quality requirement for raw water used at a bleaching plant. The quality of water has been considered a critical factor and therefore there has been no desire to depart from the standard, except for those exceptional cases when reaching the quality requirements is impossible due to the poor quality of the mill's raw water. In these cases, too, the attempt has been to get the quality of the water to be as close to the requirements set by standard as possible.
The most important component that causes brightness loss is lignin. In secondary treatment of effluent the color does not change, even though the lignin-content decreases. This has led to erroneous interpretation of the significance of color in the waters and filtrates of a pulp bleaching process.